As with all works of literature, the main theme of this work can be interprete in various ways, and there can be more than one theme. The most obvious theme seems to be spelled out overtly for the reader toward the beginning of the story: Do not interfere with fate.
In this story, three characters, Mr. and Mrs. White and their son Herbert, are visited by Sergeant-major Norris. Eventually during the visit, the seargent-major gives the family a mummified monkey's paw which he claims can grant three wishes. Before he leaves, he instructs the family to throw it into the fire and let it burn. Instead, the family wishes for 200 pounds in order to make the final payment on their house.
This wish, as well as all of the wishes in the story, are granted unconnventianally and tragically. The Whites get their 200 pounds, but it's in the form of compensation for their son's death on the job. When they wish for his life back, he does not come back as the Herbert they know. He comes back as a corpse that has been underground for days.
To readers, it seems that the Whites should have been aware of the ill fate that the monkey's paw would bring. The seargent-general warns them rather blatantly at the beginning of the story. He says, "It had a spell put on it by an old fakir... a very holy man. He wanted to show that fate ruled people's lives, and that those who interfered with it did so to their sorrow." Had the family listened to these words, they would have known that only bad could come from making three wishes to the monkey's paw. Instead, they played into the old fakir's assumptions that all men will try to interfere with fate if given the opportunity.
Another theme could be greed. The Whites are greedy for money and pay for that greed with their son's life. Then, they are greedy to have their son back from the dead without thinking what a horrible experience this will be.
This is a simple story that just has quite an important message. Jacobs shows that we should not try to change our own fate, regardless of how tempting it may be.